The invention relates generally to the copying of a colored original.
More particularly, the invention relates to a method of and an arrangement for copying a colored original line-by-line and point-by-point.
In order to reproduce a colored original, it is known to electronically scan the original line-by-line in each of the three primary colors blue, red and green. Electrical imaging signals are generated by the scanning operation, and these signals are electronically processed to enhance the final image of the original.
Such electronic image processing is being increasingly used in the production of colored positive images from colored negatives or diapositives. During image processing, the imaging signals are corrected or modified according to specific criteria. The modified imaging signals are sent to a color exposure device. The exposure device converts the electrical imaging signals back into an optical image which is exposed onto photographic copy material, e.g., color negative paper. A procedure of this type, as well as an apparatus for performing the procedure, is disclosed, for example, in "An Experimental Cathode Ray Tube Printer", Journal of Imaging Technology, Vol. 12, No. 3, pp. 135-139.
By selective adjustment of amplifiers in the three color channels, an apparatus of this type makes it possible to change the relative intensities of the different colors for purposes of correction, e.g., for the correction of color casts. The apparatus may be equipped with a color monitor and an appropriate simulator for the brightness and color gradations to be expected in the copy. In this manner, the color and density corrections made by adjustment of the amplifiers in the color channels may be immediately visualized on the monitor. However, due to the very subtle differences in shades which are required for high-quality copies, the adjustments to compensate for color casts are time-consuming and require trained personnel.
In contrast to the above, very reliable methods for automatic color correction have been developed for color copying with integral exposures. For instance, the West German Auslegeschrift No. 25 35 034 discloses a method in which photographic color negatives are examined region-by-region for the presence of color dominants. In calculating overall densities for a negative in the three primary colors, those regions containing color dominants are given less weight than the remaining regions. As used throughout the specification and claims, the phrase "less weight" includes zero weight. The relationships between the overall densities in the different primary colors are used to adjust the ratios of the primary colors in the copying light so that an area of the original which is essentially neutral gray, that is, which is free of dominants, is neutral gray in the copy.
The criteria for detecting color dominants and for adjusting the ratios of the primary colors in the copy light as a function of the relationships derived from dominant-free regions cannot be readily employed for the processing of imaging signals in color reproduction methods involving point-by-point exposures. The reason is that the number of points to be exposed is generally large, e.g., of the order of 2 million, so that these criteria consume excessive amounts of time and require excessive calculations.
The West German Offenlegungsschrift No. 34 12 723 discloses a method for setting a scanner in dependence upon an original to be reproduced. Here, the scanner is adjusted to a computer serving to calculate the exposures in the different primary colors. For optimum adjustment, the intensity at the scanner during scanning of the original is set in such a manner that the output signals of the photomultiplier for the brightest and darkest points of the original respectively correspond to the maximum and minimum input signals to the computer. This method thus functions solely to adjust the scanner to the exposure computer.